The present invention relates to a developing roller that is used in developing devices such as a copying machine, a facsimile and a laser beam printer. The present invention relates more specifically to a developing roller particularly suitable for a developing device of a nonmagnetic one-component toner-developing mode.
Formerly, the main current of the developing mode of developing devices such as a laser beam printer was a developing mode using a magnetic two-component toner. However, from the standpoint of the protection of the earth environment and saving of resources, attention has been paid to a developing mode using a nonmagnetic one-component toner, in which toner recovery is unnecessary, a waste toner is not generated and all amounts of a toner inside a toner cartridge can be used up. Therefore, efforts have been being made to put this mode into practical use.
A developing device of this nonmagnetic one-component toner developing mode basically comprises a photosensitive drum; electrostatic latent image-forming means for forming an electrostatic latent image on the photosensitive drum; a developing roller which rotates while it contacts the photosensitive drum directly or is close thereto; a toner-supplying member comprising, e.g., a supply roller made of polyurethane sponge, for supplying a nonmagnetic one-component toner to the developing roller; and a toner-regulating member comprising, e.g., a blade made of urethane rubber or urethane resin, for regulating the toner supplied onto the developing roller into a uniform thickness. In general, the electrostatic latent image-forming means forms an electrostatic latent image on the surface of the photosensitive drum on the basis of given image information. Simultaneously, the toner-supplying the developing roller. The toner-regulating member regulates the toner supplied onto the developing roller into a uniform thickness, so as to form a uniform thin layer of the toner on the surface of the developing roller. The developing roller having, on its surface, the thin toner layer of the uniform thickness causes the toner to adhere successively onto the electrostatic latent image on the photosensitive drum at a nip portion between the roller and the photosensitive drum or at a portion near the drum. In this way, the toner development is performed.
The developing roller used in the developing device of such a nonmagnetic one-component toner developing mode is a roller which causes the toner charged positively or negatively by friction contact to adhere electrostatically to the surface of the roller. This roller is composed of an electrically conductive roller. Such an electrically conductive roller usually has a structure wherein an electrically conductive layer made of an electrically conductive material is formed on the columnar surface of an electrically conductive core (metal core) constituting the body of the roller. Hitherto, as the conductive material constituting this conductive layer, there has been used an electrically conductive rubber material in which an electrically conductive material, such as carbon black or a metal powder, for imparting electrical conductivity, is blended with a base of a rubber material such as silicone rubber, acrylonitrile/butadiene rubber, urethane rubber, or silicone-modified ethylene/propylene rubber.
However, silicone rubber and silicone-modified ethylene/propylene rubber contain a low-molecular siloxane. The siloxane is transferred to the surface, resulting in a problem that the surface of the photosensitive drum contacting the developing roller is polluted by this low-molecular siloxane. As for acrylonitrile butadiene rubber, sulfur or a sulfur derivative is used as a vulcanizing agent thereof. Thus, the surface of the photosensitive drum is also polluted by the vulcanizing agent. On the other hand, urethane rubber hardly pollutes the photosensitive drum; however, its volume resistance is largely changed by a change in environment. That is, the dependency of the resistance upon the environment is large. Thus, urethane rubber is not so practically used.
In general, the roller having a rubber layer alone has a large coefficient of dynamic friction at its surface. Thus, a large torque is required for rotating the developing roller, in view of the friction between the roller and a contact member such as the toner-regulating blade or the toner-supplying roller. Thus, a stronger motor becomes necessary. This hinders downsizing of the device, and reductions in electric power consumed and costs.
Thus, in order to solve problems of conventional rubber-based conductive layers, in particular, the problem of pollution of the surface of the photosensitive drum and the problem of driving torque of the developing roller, a covering layer has been formed on the surface of the rubber-based conductive layer. For example, the gazette of Japanese Patent No. 2504978 discloses that a covering layer comprising a reaction product (fluorine-containing urethane) of an urethane resin and a fluorine-containing compound having a functional group is formed on the surface of a rubber-based conductive layer. This covering layer blocks the above-mentioned transferring pollutant in the rubber-based conductive layer thereat. Therefore, it is possible to prevent the transferring pollutant from polluting the surface of the photosensitive drum.
However, even in developing rollers having the conventional covering layer, toner is damaged since the toner is subjected to friction between the surface of the developing roller and the toner-regulating blade or the photosensitive drum. As a result, there arises the phenomenon that the toner fixes onto the surface of the developing roller, that is, the phenomenon of filming, adversely affecting the printed images. The filming occurs also on the photosensitive drum. In this case, the filming portion appears as a missing portion in the form of a stripe or stripes in the printed image. The filming is remarkably generated in a toner seal portion, which a strong stress is applied to toner. In this case, a bad effect is produced on the sealability for the toner. At worst, problems such as leakage of the toner arise.